The hydrogen sulphide content of fluids in the permeable formations of oil wells has an important impact on the economic value of the produced hydrocarbons and production operations. Typically, the sulphur content of crude oils is in the range 0.3-0.8 weight percent and the hydrogen sulphide content of natural gas is in the range 0.01-0.4 weight percent(1), although concentrations of hydrogen sulphide in natural gas of up to 30 weight percent have been reported(2) Several recent reports(3,4) have claimed a systematic increase in the sulphur content of crude oils over the past 10-20 years and anticipate further significant increases in the concentration of hydrogen sulphide in both oil and natural gas. H{dot over (a)}land et al.(5) have recently found a correlation between the hydrogen sulphide concentration of produced hydrocarbons from the Norwegian continental shelf and the reservoir temperature; above about 110° C., the hydrogen sulphide content of produced hydrocarbons was observed to increase exponentially with temperature, while below this temperature the hydrogen sulphide concentration was negligible. Orr and Sinninghe Damsté(6) have given a recent review of the geochemistry of sulphur in naturally-occurring hydrocarbons.
The presence of hydrogen sulphide in produced fluids can give rise to critical safety problems. The exposure limit recommended by the US National Institute for Occupational Health is 10 ppm per 10 minutes of exposure. The gas is immediately lethal at a concentration of about 300 ppm, which is comparable to the toxicity of hydrogen cyanide. The human nose can detect concentrations as low as 0.02 ppm and its maximum sensitivity is about 5 ppm; the nose becomes increasingly unable to detect hydrogen sulphide at concentrations of 150-200 ppm. Detection limits below about 5 ppm are therefore desirable.
The hydrogen sulphide content of oilfield brines and produced water can also give rise to significant production problems. The breakthrough of seawater during the secondary recovery of hydrocarbons can give rise to the enhanced production of hydrogen sulphide by the action of sulphate-reducing bacteria on the sulphate in the seawater. Scott and Davies(7), Aplin and Coleman(8) and Kalpakci et al.(9) have recently discussed the formation of hydrogen sulphide by the action of sulphate-reducing bacteria in oil wells which co-produce water containing high sulphate concentrations.
The hydrogen sulphide content of reservoir fluids can be determined from samples collected by wireline fluid sampling tools such as Schlumberger's Modular Dynamics Tester(10,11) or related sampling tools(12,13). Fluid samples are usually collected in metal containers, which are able to maintain the pressures at which the samples were collected. A well known problem associated with sampling fluids containing hydrogen sulphide is partial loss of the gas by reaction of the metal components, particularly those made from iron-based metals(14-16). The hydrogen sulphide gas readily forms non-volatile and insoluble metal sulphides by reaction with many metals and metal oxides, and analysis of the fluid samples can therefore give an underestimate of the true sulphide content.
It is therefore an object of this invention to describe the application of several sensors concepts for the measurement of the hydrogen sulphide concentration of samples collected by a wireline fluid sampling tool.
A large number of laboratory techniques exist for the measurement of hydrogen sulphide in samples of fluids of either geological or environmental interest, collected by sub-surface sampling tools or from fluid streams at the surface. Reservoir fluids can be collected and stored at reservoir pressures by wireline sampling tools, such as Schlumberger's MDT (Mark of Schlumberger) tool(10,11) or the single-phase hydrocarbon sampling tool described by Massie et al.(17). Burke et al.(18) have described the use of gas chromatography to analyse the samples of pressured reservoir oil samples captured by wireline sampling tools. The hydrogen sulphide contents of the oil samples, together with the concentrations of the hydrocarbons C1 to C7 and the gases oxygen, nitrogen and carbon dioxide, were measured by a gas chromatograph using a thermal conductivity detector. Cutter and coworkers(19,20) have described the use of gas chromatography with a photoionisation detector to analyse the soluble sulphide content of freshwater and marine water and sediment samples. The water samples were acidified to convert all soluble sulphide to hydrogen, sulphide, which was then stripped from solution by a stream of helium gas. Devai and DeLaune(21) have used gas chromatography to separate and quantify mixtures of sulphur-containing compounds, such as hydrogen sulphide, carbon disulphide and dimethyl sulphide. Bethea(22) has reviewed the early use of gas chromatography to analyse samples containing hydrogen sulphide.
Separation techniques have also been used to quantify the concentration of sulphide in liquid (aqueous) samples. Masselter et al.(23) have used capillary ion electrophoresis to determine the sulphide ion concentration in the liquors produced during the manufacture of paper and paper pulp. Sulphide ions were separated from other anions (chloride, sulphate, sulphite, oxalate, carbonate and thiosulphate) at a pH of 11.0 using a carrier phase consisting of sodium chromate, acetonitrile and a cationic polymer. The electrophoretic method gave a linear response between sulphide concentration and peak area over the range 1-100 ppm. Font et al.(24) have used capillary electrophoresis to determine the concentration of sulphide ions in waste water samples from the leather industry. The detection limit for sulphide ions in the effluent samples was determined to be 10 μg/l (10 ppb) using the direct absorption of ultraviolet light at a wavelength of 229 nm as the detection method. Yashin and Belyamova(25) demonstrated the use of an amperometric detector for the quantification of sulphide ions in aqueous solution by ion chromatography. Sulphide ions were readily separated from iodide and thiocyanide ions using a mobile phase consisting of sodium chloride and sodium hydrogenphosphate (pH=6.7) and an applied redox potential of 1.3 V. The detection limit of sulphide ions in this matrix was determined to be 20 μg/l (20 ppb). Hassan(26) used ion chromatography to determine the concentration of sulphide ions in aqueous solutions in the presence of sulphate, sulphite and thiosulphate ions. The mobile phase consisted of a borate-gluconate buffer (pH=8.5) containing EDTA and ascorbic acid to prevent oxidation of the sulphite ions. The ions were separated using a commercial anion exchange column and detected with either conductivity or uv/vis detectors. The detection limit of sulphide in the aqueous matrix was 10 ppm. Nagashima et al.(27) developed a liquid chromatographic method to determine the concentration of sulphide ions in human blood samples. The sulphide ions were reacted with 2-amino-5-N,N-diethylaminotoluene and Fe(III) ions under acidic conditions to form a methylene blue derivative, which was separated from the reaction mixture by liquid chromatography and detected by fluorescence (excited at λ=640 nm and detected at λ=675 nm). A linear relationship between sulphide ion concentration and fluorescence intensity was observed over the concentration range 15-1500 ng/l (0.015-1.5 ppb).
Kalpakci et al.(9) recommended that, where possible, the hydrogen sulphide content of oilfield fluid samples should be determined on site, largely to avoid the problems of loss by reaction with metal components in the sample containers. Kalpakci et al. suggested two methods to analyse the hydrogen sulphide content of gas samples. The first method used a Dräger tube in which the sample of hydrogen sulphide gas was carried by an inert gas (e.g., nitrogen) and where it reacted with a coating on the wall of the tube to produce a colour change; the length of tube showing the colour change was directly proportional to the concentration of hydrogen sulphide(28,29). The second method used specific hydrogen sulphide gas sensors and these may be either solid-state metal oxide sensors (see refs. 30-33 , for example), surface acoustic wave(34) and electrochemical sensors(35,36). These methods can also be used to determine the hydrogen sulphide content of liquid samples which are purged with a stream of inert gas; the pH of aqueous samples must be less than a value of 5 to ensure all soluble sulphides exist as hydrogen sulphide.
Electrochemical methods, particularly potentiometric methods, have been used widely to determine the concentration of sulphide dissolved in aqueous solutions. Silver/silver chloride electrodes have been used for many years to measure the concentration of sulphide ions (HS− and S2−) in aqueous media at pH values typically in the range 7-12(37-42). Hu and Leng(43) have used a carbon paste electrode prepared with diisooctyl phthalate to determine the concentration of sulphide ions (HS−) in aqueous solutions buffered to a pH value of 9.00 using sodium tetraborate. The carbon paste electrode gave responses of 135-180 mV/decade and 40-60 mV/decade over the concentration ranges 1.5×10−7-2.5×10−6 molar (5-85 ppb) and 7.0×10−6-1.0×10−3 molar (0.24-34 ppm), respectively, using a saturated calomel reference electrode. The response of the carbon paste electrode was therefore considerably greater than the Nernstian response (30 mV/decade) of a conventional silver/silver sulphide electrode. Hu and Leng observed that the response of the carbon paste electrode to sulphide ions showed no significant dependence on the concentration of cyanide and iodide ions, in contrast to conventional silver/silver sulphide electrodes. Hadden(44) has described the use of combined silver/silver sulphide and pH electrodes to monitor the soluble sulphide content in water-based drilling fluids. Jeroschewski et al.(45) have described an amperometric gas sensor to determine the concentration of hydrogen sulphide in aqueous media; the hydrogen sulphide diffused from the aqueous solution through a PTFE membrane and into an inner aqueous solution where it dissociated to form HS− ions, which were subsequently oxidised by ferricyanide ions. Ma et al.(46) have described a potentiometric method of measuring the concentration of HS− ions in aqueous solutions using a membrane electrode produced by the electropolymerisation of binaphthyl-20-crown-6. The electrode showed typical linear behaviour over the concentration range 2×10−7 to 2×10−5 molar (7-700 ppb) and a detection limit of about 5×10−8 molar (2 ppb) at a pH value of 7.5. Atta et al.(47) have developed a sulphide selective electrode formed by electrochemically depositing a film of poly(3-methylthiophene) and poly(dibenzo-18-crown-6) on a metal alloy electrode. The electrode gave an approximately Nernstian response over the range of sulphide ion concentration of 1.0×10−7 to 1.0×10−2 molar (3 ppb-320 ppm) and over the temperature range 10-40° C. Surprisingly, the electrode response showed little variation over the pH range 1-13 for sulphide ion concentrations in excess of about 10−5 molar.
Opekar and Bruckenstein(48) have developed a cathode stripping voltammetry technique to determine the concentration of hydrogen sulphide in a flowing stream of gas. The hydrogen sulphide was reacted with silver metal deposited in a porous PTFE membrane under a constant potential of −0.2 V, measured with respect to the saturated calomel electrode. Silver sulphide was formed in the membrane with the gas flowed at a known flow rate for a fixed period of time. The silver sulphide was removed from the electrode at a fixed potential of −0.9 V (with respect to the saturated calomel electrode) using a high flow rate of sulphide-free nitrogen (or air); the measured current was observed to be linear in the hydrogen sulphide concentration over the range 2.5-18 ppb. Kirchnervona et al.(49) fabricated a potentiometric hydrogen sulphide gas sensor for use in the temperature range 635-770° C. The potentiometric sensor consisted of a silver -□- alumina membrane with a silver reference electrode and a silver sulphide/molybdenum sulphide working electrode. The sensor measured the activity of elemental sulphur in the gas phase in an inert carrier gas (e.g., nitrogen) and at high temperatures this is provided by the dissociation of hydrogen sulphide. The low thermal stability of silver sulphide limited the detection limit for hydrogen sulphide to 10 ppm.
Numerous optical and wet chemical methods exist to measure the concentration of hydrogen sulphide, either in gaseous form or in aqueous solutions. Some of the classical wet chemical methods to determine the concentration of hydrogen sulphide in aqueous solutions have been compared by Bethea(22).
Weldon et al.(50) have developed a spectrophotometric method for measuring the concentration of hydrogen sulphide using the near-infrared absorption of the S—H combination band at a wavelength of 1590 nm. The near-infrared source was a distributed feedback laser and concentrations as low as 10 ppm at ambient pressures could be measured with an optical path length of 5 meters. Smits et al.(51) have described a near-infrared spectrometer for use in a wireline fluid sampling tool to differentiate hydrocarbon and brine samples and to detect the presence of gas. The optical path length of the spectrometer is of the order of 1 mm, which is insufficient to allow the detection and quantification of hydrogen sulphide in most wellbore fluid samples. Arowolo and Cresser(52) developed a method to measure the concentration of hydrogen sulphide in aqueous solutions by extracting the gas after acidifying the test solution with 3 molar hydrochloric acid and measuring the optical density (absorption) of the gas in an optical cell at a wavelength of 200 nm. An optical pathlength of 13 cm allowed a detection limit of 60 ppb sulphide in aqueous solutions; a linear relationship between absorbance and hydrogen sulphide concentration was observed up to a sulphide concentration of 100 ppm. Howard and Yeh(53) have developed a similar technique to determine the sulphide ion concentration of aqueous samples, although the detection system was based on a flame photometric detector. The flame emission was measured using a broad-band photomultiplier tube, which enabled a detection limit of 70 ppb to be achieved for sulphide dissolved in water. Over the concentration range 200-1700 ppb the photomultiplier output increased with the square of the concentration of sulphide in aqueous solutions.
Saltzman and Leonard(54) have described the use of a diode array ultraviolet/visible spectrophotometer to measure the concentration of various sulphur-containing gases, including hydrogen sulphide; the spectrophotometer is commercially available and manufactured by Ametek® (Newark, Del., USA). Suleimenov and Seward(55) have measured the far-ultraviolet spectra of aqueous solutions of hydrosulphide ions (HS−) over the temperature range 25-350° C. at the saturated vapour pressure of water; the intense spectrum arises from charge transfer processes between HS− and water. Parks(56) has described a method of measuring the concentration of hydrogen sulphide by reaction with ozone to generate an electronically excited state of sulphur dioxide, which decayed to the ground state by the emission of radiation. The integrated intensity of the chemiluminescence was used to determine the concentration of hydrogen sulphide.
Hager(57) has described methods of sampling hydrocarbons and selected chemicals from drilling fluids during the drilling process and measurement of their concentrations using fluorescence or absorption spectroscopy. The drilling fluid samples, which contain the chemical species derived from the drilled formations, are captured through a membrane filter, located in the bottom hole assembly as part of a measurement-while-drilling tool string, and transported to the optical detection system using a suitable solvent. Hager did not specify hydrogen sulphide as a chemical species analysed in the drilling fluid.
The concentration of hydrogen sulphide in aqueous solution has also been determined spectrophotometrically using the reaction between hydrogen sulphide and a mixture of iron(III) chloride and N,N-dimethyl-p-phenylenediamine to generate the dye methylene blue which can be determined spectrophotometrically at a wavelength of 660 nm (58-60). Habicht and Canfield(61) have used the methylene blue spectrophotometric technique to quantify the hydrogen sulphide content of microbe-rich sediments from several locations. Spaziani et al.(62) have recently described an on-line method to measure the concentration of sulphide ions by detecting the formation of methylene blue by fluorescence using a diode laser excitation source. Alternative spectrophotometric techniques using methylene blue have been used to measure the concentration of sulphide in aqueous solution. Phillips et al.(63) have measured the concentration of hydrogen sulphide in surface waters and the interstitial water in near-surface sediments using the formation of methylene blue. The methylene blue was determined spectrophotometrically at a wavelength of 680 nm and the detection limit for total sulphide content was 0.01 mg/l (10 ppb). Koh et al. (64) have described a method for measuring the concentration of sulphide ions (S2−) from the formation of thiocyanide ions (SCN−) in aqueous solution, by reaction with cyanide ions and hydrogen peroxide, and subsequent extraction of thiocyanide into 1,2-dichloroethane using methylene blue to form an ion pair. The methylene blue-thiocyanide ion pair was detected by a spectrophotometer operating at a wavelength of 657 nm. Mousavi and Sarlack(65) have used the reduction of methylene blue by hydrogen sulphide ions (HS−) using tellurium(IV) ions as a catalyst. The reduced methylene blue species is colourless and the concentration of hydrogen sulphide ions was determined by the loss in the absorbance of methylene blue measured at a wavelength of 663 nm. Shanthi and Balusubramanian(66) described a spectrophotometric method to measure low concentrations of hydrogen sulphide using its oxidation of bromate ions to bromine, which subsequently reacted with the indicator 2′,7′-dichlorofluorescein to form a dibromo compound detected at a wavelength of 535 nm.
Narayanaswamy and Sevilla(67) have described a detector for hydrogen sulphide in the gas phase that was based on the change in the reflectivity of paper soaked in lead acetate exposed to the gas. The change in the reflectivity of the paper at 580 nm gave the greatest sensitivity and allowed gas phase concentrations as low as 50 ppb to be determined. The change in reflectivity of the lead acetate paper after a fixed time of 10 seconds was found to give accurate and reproducible measurements of hydrogen sulphide concentration. Neihof(68) reported on the use of filter paper impregnated with lead acetate to determine the concentration of hydrogen sulphide in seawater containing a fire fighting foam. The concentration of hydrogen sulphide was estimated by human observation of the colour of the paper: barely detectable coloration at 2 ppm, darkening at 4 ppm, light brown coloration at 8 ppm which turned to dark brown at 20 ppm. The lead acetate paper was protected from direct contact with the seawater sample by the use of a silicone polymer film, which enabled the transport of the hydrogen sulphide to the lead acetate paper but not the seawater. Neihof(68) also described the use of lead acetate powder immobilised in a cured silicone polymer to estimate the concentration of hydrogen sulphide in crude oil samples by a colorimetric test. The silicone polymer allowed hydrogen sulphide to reach the lead acetate particles but not the liquid hydrocarbon. A second colorimetric indicator for hydrogen sulphide, a mixture of anhydrous copper sulphate and copper thiocyanate, was also immobilised in a silicone polymer film. The indicator became an increasingly intense grey-green colour as the hydrogen sulphide content of seawater samples was increased from 2 to 32 ppm.
Eroglu et al.(69) used the luminescence of the cadmium sulphide (CdS) particles formed when hydrogen sulphide in a gas stream contacted cadmium salts, such as cadmium chloride and cadmium acetate, on paper and various polymer surfaces. The excitation of the cadmium sulphide formed on the surfaces in the spectral region 300-350 nm gave well-defined emission spectra in the region 400-750 nm. The luminescence intensity was linear in the concentration of hydrogen sulphide in the range 0.03-3 ppm. Volkan et al. (70) constructed a sensor to measure the hydrogen sulphide content of air using a flow tube whose walls were coated with silica gel treated with cadmium chloride. The length of the cadmium sulphide spot, which was determined by fluorescence excited by radiation at a wavelength of 300 nm, was found to be linear in the hydrogen sulphide concentration in the air over the concentration range 0.2-1.3 ppm. Cardoso et al. (71) have developed a detector for atmospheric hydrogen sulphide using the quenching of the fluorescence of alkaline fluorescein mercuric acetate. The hydrogen sulphide reacted with the fluorescein mercuric acetate solution on a small drop attached to an optical fibre, which excited the solution at a wavelength of 495 nm; a small silicon photodiode was used to measure the fluorescence at a wavelength of 530 nm. The maximum volume of the liquid drop was 60 μl and controlled detachment of the drop enabled a new sensing surface to be exposed to the hydrogen sulphide. The fluorescence detector was capable of detecting hydrogen sulphide in flowing air samples at concentrations of 30 ppb (by volume) with response times of less than 4 minutes. Choi(72) has described the fabrication of a reversible fluorescence sulphide ion sensor for aqueous solutions based on the fluorescence quenching of tetraoctylammonium fluorescein mercuric acetate. The tetraoctylammonium fluorescein mercuric acetate indicator was immobilised in an ethyl cellulose membrane formed on a transparent plastic sheet and located on the wall of a glass flow cell. The fluorescence spectra had a peak intensity at 536 nm and the fluorescence was observed to respond to hydrosulphide ions (HS−) over the concentration range 0.012−120×10−6 mole/L (0.4 ppb-4 ppm) for aqueous solutions in the pH range 9.0-12.5. The tetraoctylammonium fluorescein mercuric acetate fluorescence sensor could be regenerated by rinsing with a solution containing sodium acetate and sodium hypochlorite.
Lessard and Ramesh(73) have described a method of measuring the concentration of hydrogen sulphide (or sulphide ions) by reaction with scavenging reagents whose fluorescence properties are subsequently changed. Non-fluorescent amines with the general structure R—NH—CH2—NH—R′, where R and R′ are groups which contain electronically-active units, react with hydrogen sulphide (or sulphide ions) to form molecules of the form R—NH—CH2—S—S—CH2—NH—R′ which are fluorescent. For example, the amine 6-aminoquinoline was reacted with formaldehyde to generate a non-fluorescent diamine that reacted quantitatively with hydrogen sulphide to form a fluorescent disulphide. Both water- and oil-soluble diamines have been synthesised; for example, a water-soluble sulphide scavenger was synthesised by coupling two molecules of morpholine with formaldehyde. Lessard and Ramesh(73) showed that some maleimides can be made fluorescent by reaction with hydrogen sulphide or sulphide ions. For example, N—(1-pyrene)maleimide is not fluorescent but its reaction product with hydrogen sulphide is fluorescent. Lessard and Ramesh also demonstrated that some diamines exhibit fluorescence that is quenched by reaction with hydrogen sulphide. For example, the amine 6-aminocoumarin can be coupled using formaldehyde to form a fluorescent diamine; reaction with hydrogen sulphide forms a disulphide, which exhibits no fluorescence.
McCulloch et al.(74) described in some detail the use of optical waveguide sensors to measure the concentration of hydrogen sulphide. Novel waveguide sensors for hydrogen sulphide were developed using both optical absorption and Raman scattering detection techniques. A sol-gel coating technique was used to deposit ferrocene, in the form of the ferricenium cation [(C5H5)2Fe(III)]+, on an exposed portion of optical fibre. The ferricenium cation was reduced in the presence of hydrogen sulphide and its colour changed from blue-green to orange. The colour change was monitored at a wavelength of 620 nm. The ferrocene was oxidised back to the initial ferricenium complex by exposure to air or oxygen. Surface-enhanced resonance Raman spectroscopy was used to detect the presence of hydrogen sulphide using methylene blue adsorbed on a film of silver deposited on an optical fibre in the presence of ammonium molybdate. A recent patent application(75) has described the use of optical fibre sensors to measure the concentration of a number of chemical components in drilling fluids, including hydrogen sulphide, in the downhole environment while drilling. The sensing element attached to the optical fibre was described as a suitable colorimetric indicator immobilised in a porous glass matrix generated by a sol-gel process. A specific calorimetric indicator for the measurement of the concentration of hydrogen sulphide was not disclosed.
The application of equilibrium headspace analysis to the quantitative determination of gases in solution is a well-known technique, particularly for gas chromatography(76-78). Vitenberg et al.(79) and Brunner et al.(80) have described the quantitative analysis of hydrogen sulphide in aqueous media by headspace gas chromatography. Ramstad et al.(81) have used headspace gas chromatography to detect the evolution of hydrogen sulphide from dry powder samples. Kolb and Ettre(82) have explained a procedure for the analysis of the hydrogen sulphide content of crude oil by headspace gas chromatography. The use of the equilibrium headspace technique with specific gas sensors to determine the gas content of liquid samples does not appear to have been reported in the open literature.
The determination of the bubble point of liquid hydrocarbon samples and their gas content in a wellbore under reservoir conditions using a wireline tool has been described in two separate patents(83,84). Schultz and Bohan(83) have described in some detail the design of a wireline tool that captures a sample of liquid hydrocarbon with the purpose of expanding it to produce gas. The measured pressure-volume relationship obtained during the expansion allows the compressibility and the bubble point of the liquid hydrocarbon to be determined; the bubble point is readily determined from the rapid change of slope in the pressure-volume curve. A more recent patent has been assigned to Yesudas et al.(84) who described a pressure-volume technique for determining the compressibility and the bubble point of samples of liquid hydrocarbons captured by a wireline (or similar) sampling tool. The volume of gas dissolved in the sample could also be measured to determine the gas-oil ratio (GOR). Neither of these two patents gives any description of any technique to measure the concentration of any liquid hydrocarbon or exsolved gas samples. In addition, neither patent gives any description of measurements of the gas content of captured water samples.
Kurosawa et al.(85) have described the construction of a biosensor for the determination of the hydrogen sulphide content of aqueous samples. The sensor consisted of cells of the bacterium Thiobacillus thiooxidans immobilised in a porous filter in an oxygen electrode. The bacterium oxidised hydrogen sulphide in aqueous solution and decreased the oxygen content that was detected by the oxygen electrode. Concentrations of hydrogen sulphide in water of 5×10−5 molar (1.7 ppm) were detected with a response time of about 5 minutes.
There appear to be no reports in the public domain which describe the measurement of the concentration of hydrogen sulphide, or any other chemical species, in samples of hydrocarbon or water captured by a wireline sampling tool using any specific chemical sensor or detector system. Mariani and Mullins(86) have discussed the use of microwave (molecular rotation) spectroscopy to measure the concentration of hydrogen sulphide extracted as a gas from sub-surface fluid samples. The design of a downhole microwave spectrometer, operating the frequency range 150-400 GHz, was outlined, including the use of a Fabry-Perot interferometer to replace a long path length gas cell. Mariani and Mullins discussed the measurement of hydrogen sulphide in a wireline fluid sampling tool and a measurement while drilling technique. The United Kingdom Patent No. 2344365 B described a method of extracting the hydrogen sulphide from hydrocarbon samples using a packed bed of a metal oxide and discussed the possibility of monitoring the changes in the electrical conductivity of the metal oxide to measure the concentration of removed hydrogen sulphide. Hager(57) has described methods for sampling hydrocarbons and other chemical species from the drilling fluid during the drilling process using a sampling tool close to the bit; hydrogen sulphide was not specifically identified as a chemical species. A recent patent application by Weirich et al.(75) has described the application of an optical fibre-based sensor to measure the concentration of hydrogen sulphide in drilling fluid close to the bit during drilling. This patent application appears to be the only prior art for any measurement of hydrogen sulphide in the wellbore environment.